The present invention relates to a damping element for a wind turbine rotor blade, a wind turbine rotor blade including such a damping element and a wind turbine equipped with a wind turbine rotor blade having a damping element.
Wind turbine rotor blades are used to extract kinetic energy from the wind and to convert this kinetic energy into rotational energy of a driving shaft. Typically, the driving shaft is connected to an electric generator for power generation but may also be used to drive a pump, a mill work or the like. In any case, the rotor blades are subject to high loads caused by short-term fluctuations of wind speed. In particular, wind gust loads affect the structural blade design and permanent wind turbulence provides a substantial contribution to material fatigue of the rotor blades.
To withstand the above-described wind gust loads, wind turbine rotor blades are typically made from fiber-reinforced plastics with a polymer resin matrix. Some blade designs include also foam layers or a wooden core. Thus, the typical blade design and blade material composition exhibits a flexible construction with low structural damping, i.e. below about 1%. Typically, the flexible design results in first bending and/or torsional modes of the blade in a range of only a few Hertz. Furthermore, the low structural damping of the blade material is not sufficient for dampening such bending and/or torsional modes of the blade. Therefore, the rotor blades typically have high dynamic response so that the low bending and/or torsional modes of the blades are excited during operation. Thus, the high flexibility and low internal modal damping results in increased fatigue of the blade due to internal stresses. As a result, the lifetime of the blades is reduced by the bending and torsion thereof and fatigue fractures are likely to occur. Current approaches to cope with this problem use either high-strength materials to increase the blade stiffness and/or simply more material to increase structural damping. However, high-strength materials are relatively expensive and the use of more material leads to a considerable increase in blade mass.